This story exists in a world where everyone is classified either as a little or a big. Upon birth, doctors do quick exams on infants to determine the baby's classification and from then on, it's tied to their identities. It was nearly impossible for anyone to hide their status because for one, they're registered at birth. Second, as people grow up it becomes even easier to spot who's a little, and who's a big.
On a physical level, it was quite easy to tell bigs and littles apart:
Bigs develop and grow up like how people were expected to before the outbreak of bigs and littles. Littles, on the other hand are a different story: They don't develop normally in a physical sense. Girls for instance, never have their cycle and don't really develop more than little breast buds. Naturally, littles also lean towards the smaller side of the spectrum and have younger-looking features: Some would even be described as baby-faced. They weren't necessarily child-sized (though, quite a few were), but it was rare to see a little be taller than a big of the same age group.
Psychologically, there isn't much of a difference between the two groups:
When not dropped to their little age, littles typically exhibit the same emotional threshold as their peers do. There are some special cases, for instance toddler and baby littles, who may be on the more emotional side even when aged; but other than that there's not much of a significant difference. Generally everyone reaches cognitive developmental milestones at the same age regardless of their classification.
We'll learn even more about bigs and littles as the story progresses. For now, let's meet our (main and eventually recurring) characters:
Bigs:Steve Rogers
Tony Stark
Pepper Potts
Sam Wilson
Bucky Barnes
Bruce Banner
Phil Coulson
Melinda May
Leopold Fitz
Jemma Simmons
Maria Hill
Nick Fury
Littles:
Natasha Romanoff - to be determined
Clint Barton - 12
Wanda Maximoff - 7
Daisy Johnson - 6
Lincoln Campbell - 11
Bobbi Morse - 10